For a while, it looked to me like my bicycle's rear-light was
“somewhat” dim. I supposed that the batteries might be getting low,
but I was not sure.

So, given that I have a multimeter, and that the internets give ~20%
decrease in voltage as reasonable limit for a non-recheargable battery
that is not "dead", I set to measure. The batteries are type
N Battery (which were not as common as I hoped, at
least not as common as either AA, AAA, D or C, all of which I
could find at the nearby store, but not N).